Background: There is limited evidence exploring sibling's perspective in Prader-Willi syndrome research. Objectives: To investigate the experiences and support needs of the siblings of individuals with Prader-Willi syndrome. Methods: This two-stage qualitative study involved siblings aged 11 years onwards (n = 11) and parents (n = 8). Stage 1 utilised multiple age-specific focus groups and paired interviews. Data were analysed through integrated discourse and thematic analysis. Stage 2 included consensus meetings utilising the Nominal Group Technique. Results: Five major themes emerged during Stage 1: family and relationships, food practices, shared understanding, adjustment and advocacy, and support needs. The findings revealed some commonalities in the siblings' discourse, although their experiences varied based on their age. Parents were aware of these differences but had a different focus. Conclusions and Implications: Sibling's experiences show a developmental trajectory and could explain variable parental perspectives. A lifespan and family-focused view could help tailor support systems.